We are told in the Torah in several locations that we must eat unleavened bread for 7 days, put all leaven out of our homes to the extent that it is not seen anywhere near us for the full week of the festival of Pesach. We are also told in these same passages that we are to bake and eat matzah on the holiday. This begs the question of what is leaven/chametz that we must be so careful that we not eat it nor have it in our very presence.